The description of prior art will be primarily based on the references listed below:
List of References:
    1. Aruna Zhamu and Bor Z. Jang, “Hybrid Nano Filament Anode Compositions for Lithium Ion Batteries,” U.S. patent application Ser. No. 12/006,209 (Jan. 2, 2008).    2. Aruna Zhamu and Bor Z. Jang, “Hybrid Nano Filament Cathode Compositions for Lithium Ion and Lithium Metal Batteries,” U.S. patent application Ser. No. 12/009,259 (Jan. 18, 2008).    3. Kang, K., Meng, Y. S., Breger, J., Grey, C. P. & Ceder, G. Electrodes with high power and high capacity for rechargeable lithium batteries. Science 311, 977-980 (2006).    4. Rauh, R. D., Abraham, K. M., Pearson, G. F., Surprenant, J. K. & Brummer, S. B. A lithium/dissolved sulfur battery with an organic electrolyte. J. Electrochem. Soc. 126, 523-527 (1979).    5. Shim, J., Striebel, K. A. & Cairns, E. J. The lithium/sulfur rechargeable cell. J. Electrochem. Soc. 149, A1321-A1325 (2002).    6. Chu, M.-Y. Rechargeable positive electrodes. U.S. Pat. No. 5,686,201 (1997).    7. Peramunage, D. & Licht, S. A solid sulfur cathode for aqueous batteries. Science 261, 1029-1032 (1993).    8. Cunningham, P. T., Johnson, S. A. & Cairns, E. J. Phase equilibria in lithium—chalcogen systems: Lithium—sulfur. J. Electrochem. Soc. 119, 1448-1450 (1972).    9. Choi, J.-W. et al. Rechargeable lithium/sulfur battery with suitable mixed liquid electrolytes. Electrochim. Acta 52, 2075-2082 (2007).    10. Rauh, R. D., Shuker, F. S., Marston, J. M. & Brummer, S. B. Formation of lithium polysulfides in aprotic media. J. Inorg. Nucl. Chem. 39, 1761-1766 (1977).    11. Cheon, S.-E. et al. Rechargeable lithium sulfur battery II. Rate capability and cycle characteristics. J. Electrochem. Soc. 150, A800-A805 (2003).    12. Shin, J. H. & Cairns, E. J. Characterization of N-methyl-N-butylpyrrolidinium bis(trifluoro-methanesulfonyl)imide-LiTFSI-tetra(ethylene glycol) dimethyl ether mixtures as a Li metal cell electrolyte. J. Electrochem. Soc. 155, A368-A373 (2008).    13. Yuan, L. X. et al. Improved dischargeability and reversibility of sulfur cathode in a novel ionic liquid electrolyte. Electrochem. Commun. 8, 610-614 (2006).    14. Ryu, H.-S. et al. Discharge behavior of lithium/sulfur cell with TEGDME based electrolyte at low temperature. J. Power Sources 163, 201-206 (2006).    15. Wang, J. et al. Sulfur-mesoporous carbon composites in conjunction with a novel ionic liquid electrolyte for lithium rechargeable batteries. Carbon 46, 229-235 (2008).    16. Chung, K.-I., Kim, W.-S. & Choi, Y.-K. Lithium phosphorous oxynitride as a passive layer for anodes in lithium secondary batteries. J. Electroanal. Chem. 566, 263-267 (2004).    17. Visco, S. J., Nimon, Y. S. & Katz, B. D. Ionically conductive composites for protection of active metal anodes. U.S. Pat. No. 7,282,296, October 16 (2007).    18. Skotheim, T. A., Sheehan, C. J., Mikhaylik, Y. V. & Affinito, J. Lithium anodes for electrochemical cells. U.S. Pat. No. 7,247,408, July 24 (2007).    19. Akridge, J. R., Mikhaylik, Y. V. & White, N. Li/S fundamental chemistry and application to high-performance rechargeable batteries. Solid State Ion. 175, 243-245 (2004).    20. Mikhaylik, Y. V. & Akridge, J. R. Low temperature performance of Li/S batteries. J. Electrochem. Soc. 150, A306-A311 (2003).    21. Zheng, W., Liu, Y. W., Hu, X. G. & Zhang, C. F. Novel nanosized adsorbing sulfur composite cathode materials for the advanced secondary lithium batteries. Electrochim. Acta 51, 1330-1335 (2006).    22. Cheon, S.-E. et al. Capacity fading mechanisms on cycling a high-capacity secondary sulfur cathode. J. Electrochem. Soc. 151, A2067-A2073 (2004).    23. Song, M.-S. et al. Effects of nanosized adsorbing material on electrochemical properties of sulfur cathode for Li/S secondary batteries. J. Electrochem. Soc. 151, A791-A795 (2004).    24. Kobayashi, T. et al. All solid-state battery with sulfur electrode and thio-LISICON electrolyte. J. Power Sources 182, 621 (2008).    25. Wang, J., Yang, J., Xie, J. & Xu, N. A novel conductive polymer-sulfur composite cathode material for rechargeable lithium batteries. Adv. Mater. 14, 963-965 (2002).    26. Xiulei Ji, Kyu Tae Lee, & Linda F. Nazar, “A highly ordered nanostructured carbon—sulphur cathode for lithium—sulphur batteries,” Nature Materials 8, 500-506 (2009).    27. D. J. Burton, et al, “Method of Depositing Silicon on Carbon Materials and Forming an Anode for Use in Lithium Ion Batteries,” US Pub No. 2008/0261116 (Oct. 23, 2008).    28. D. W. Firsich, “Silicon-Modified Nanofiber Paper As an Anode Material for a Lithium Ion Battery,” US Patent Publication 2009/0068553 (Mar. 23, 2009).Lithium Metal Secondary Batteries:
Lithium-ion (Li-ion), lithium metal, and Li metal-air batteries are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium as a metal element has the highest capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound as an anode active material (except Li4.4Si, which has a specific capacity of 4,200 mAh/g). Hence, in general, Li metal batteries have a significantly higher energy density and power density than lithium ion batteries.
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds having high specific capacities, such as TiS2, MoS2, MnO2, CoO2, and V2O5, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode to the cathode through the electrolyte and the cathode became lithiated. Unfortunately, upon cycling, the lithium metal resulted in the formation of dendrites at the anode that ultimately caused unsafe conditions in the battery. As a result, the production of these types of secondary batteries was stopped in the early 1990's giving ways to lithium-ion batteries.
Even now, cycling stability and safety concerns remain the primary factors preventing the further commercialization of Li metal batteries for EV, HEV, and microelectronic device applications. Specific cycling stability and safety issues of lithium metal rechargeable batteries are primarily related to the high tendency for Li to form dendrite structures during repeated charge-discharge cycles or an overcharge, leading to internal electrical shorting and thermal runaway.
Many attempts have been made to address the dendrite-related issues. However, despite these earlier efforts, no rechargeable Li metal batteries have yet succeeded in the market place. This is likely due to the notion that these prior art approaches still have major deficiencies. For instance, in several cases, the anode or electrolyte structures are too complex. In others, the materials are too costly or the processes for making these materials are too laborious or difficult. An urgent need exists for a simpler, more cost-effective, and easier to implement approach to preventing Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal batteries and other rechargeable batteries.
Lithium Ion Secondary Batteries:
Parallel to these efforts and prompted by the aforementioned concerns over the safety of earlier lithium metal secondary batteries led to the development of lithium ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials as the anode. The carbonaceous material absorbs lithium (through intercalation of lithium ions or atoms between graphene planes, for instance) and desorbs lithium ions during the re-charge and discharge phases, respectively, of the lithium ion battery operation. The carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1. In order to minimize the loss in energy density due to this replacement, x in LixC6 must be maximized and the irreversible capacity loss Qir in the first charge of the battery must be minimized.
The maximum amount of lithium that can be reversibly intercalated into the interstices between graphene planes of a perfect graphite crystal is generally believed to occur in a graphite intercalation compound represented by LixC6 (x=1), corresponding to a theoretical specific capacity of 372 mAh/g. In other graphitized carbon materials than pure graphite crystals, there exists a certain amount of graphite crystallites dispersed in or bonded by an amorphous or disordered carbon matrix phase. The amorphous phase typically can store lithium to a specific capacity level higher than 372 mAh/g, up to 700 mAh/g in some cases, although a specific capacity higher than 1,000 mAh/g has been sporadically reported. Hence, the magnitude of x in a carbonaceous material LixC6 varies with the proportion of graphite crystallites and can be manipulated by using different processing conditions. An amorphous carbon phase alone tends to exhibit a low electrical conductivity (high charge transfer resistance) and, hence, a high polarization or internal power loss. Conventional amorphous carbon-based anode materials also tend to give rise to a high irreversible capacity.
In addition to carbon- or graphite-based anode materials, other inorganic materials that have been evaluated for potential lithium ion anode applications include metal oxides, metal nitrides, metal sulfides, and a range of metals, metal alloys, and intermetallic compounds that can accommodate lithium atoms/ions. In particular, lithium alloys having a composition formula of LiaA (A is a metal such as Al, and “a” satisfies 0<a≦5 when the battery is fully charged) has been investigated as potential anode materials. This class of anode material has a higher theoretical capacity, e.g., Li4Si (3,829 mAh/g), Li4.4Si (4,200 mAh/g), Li4.4Ge (1,623 mAh/g), Li4.4Sn (993 mAh/g), Li3Cd (715 mAh/g), Li3Sb (660 mAh/g), Li4.4Pb (569 mAh/g), LiZn (410 mAh/g), and Li3Bi (385 mAh/g). However, for the anodes composed of these materials, pulverization (fragmentation of the alloy particles) proceeds with the progress of the charging and discharging cycles due to expansion and contraction of the anode during the absorption and desorption of the lithium ions. The expansion and contraction also tend to result in reduction in or loss of particle-to-particle contacts or contacts between the anode and its current collector. These adverse effects result in a significantly shortened charge-discharge cycle life.
To overcome the problems associated with such mechanical degradation, composites composed of small, electrochemically active particles supported by less active or non-active matrices have been proposed for use as an anode material. Examples of these active particles are Si, Sn, and SnO2. However, most of prior art composite electrodes have deficiencies in some ways, e.g., in most cases, less than satisfactory reversible capacity, poor cycling stability, high irreversible capacity, ineffectiveness in reducing the internal stress or strain during the lithium ion insertion and extraction cycles, and some undesirable side effects.
Highest Specific Capacity Anode and Cathode Materials
Lithium-ion (Li-ion) batteries are promising energy storage devices for electric drive vehicles. However, state-of-the-art Li-ion batteries have yet to meet the cost and performance targets. Li-ion cells typically use a lithium transition-metal oxide or phosphate as a positive electrode (cathode) that de/re-intercalates Li+ at a high potential with respect to the carbon negative electrode (anode). The specific capacity of lithium transition-metal oxide or phosphate based cathode active material is typically in the range of 140-170 mAh/g. As a result, the specific energy of commercially available Li-ion cells is typically in the range of 120-220 Wh/kg, most typically 150-180 Wh/kg. Further, the current Li-sulfur products of Sion Power Co. and Polyplus Fuels Co., the two industry leaders in sulfur cathode technology, have a maximum specific energy of 400 Wh/kg.
With the rapid development of hybrid (HEV) and plug-in hybrid electric vehicles (HEV), there is an urgent need for anode and cathode materials that provide a rechargeable battery with a significantly higher specific energy, higher energy density, higher rate capability, long cycle life, and safety. One of the most promising energy storage devices is the lithium—sulfur (Li—S) cell since the theoretical capacity of Li is 3,861 mAh/g and that of S is 1,675 mAh/g. In its simplest form, a Li—S cell consists of elemental sulfur as the positive electrode and lithium as the negative electrode [Ref. 4,5]. The lithium—sulfur cell operates with a redox couple, described by the reaction S8+16Li8Li2S that lies near 2.2 V with respect to Li+/Lio. This electrochemical potential is approximately ⅔ of that exhibited by conventional positive electrodes. However, this shortcoming is offset by the very high theoretical capacities of both Li and S. Thus, compared with conventional intercalation-based Li-ion batteries, Li—S cells have the opportunity to provide a significantly higher energy density (a product of capacity and voltage). Values can approach 2,500 Wh/kg or 2,800 Wh/l on a weight or volume basis, respectively, assuming complete reaction to Li2S [Ref 6,7]. However, the current Li-sulfur products of Sion Power Co. and Polyplus Fuels Co., the two industry leaders in sulfur cathode technology, have a maximum specific energy of 400 Wh/kg.
In summary, despite its considerable advantages, the Li—S cell has been plagued with several problems that have hindered its widespread commercialization:    (1) Conventional lithium metal cells still have dendrite formation and related internal shorting issues;    (2) Sulfur or sulfur-containing organic compounds are highly insulating, both electrically and ionically. To enable a reversible electrochemical reaction at high current rates, the sulfur must maintain intimate contact with an electrically conductive additive. Various carbon—sulfur composites have been used for this purpose, but only with limited success owing to the scale of the contact area. Typical reported capacities are between 300 and 550 mAh/g at moderate rates [Ref 8].    (3) To make a sulfur-containing cathode ionically conductive, liquid electrolytes are used to serve not only as a charge transport medium but also as ionic conductors within the sulfur-containing cathode [Ref 9]. This presents difficulties of electrolyte access.    (4) The cell tends to exhibit significant capacity degradation on repeated discharge—charge cycling. This is mainly due to the high solubility of the polysulfide anions formed as reaction intermediates during both discharge and charge processes in the polar organic solvents used in electrolytes [Ref. 10]. During cycling, the polysulfide anions can migrate through the separator to the Li negative electrode whereupon they are reduced to solid precipitates (Li2S2 and/or Li2S), causing active mass loss. In addition, the solid product that precipitates on the surface of the positive electrode during discharge becomes electrochemically irreversible, which also contributes to active mass loss [Ref. 11]. To put it in a broader context, a significant drawback with cells containing cathodes comprising elemental sulfur, organosulfur and carbon-sulfur materials relates to the dissolution and excessive out-diffusion of soluble sulfides, polysulfides, organo-sulfides, carbon-sulfides and/or carbon-polysulfides (hereinafter referred to as anionic reduction products) from the cathode into the rest of the cell. This process leads to several problems: high self-discharge rates, loss of cathode capacity, corrosion of current collectors and electrical leads leading to loss of electrical contact to active cell components, fouling of the anode surface giving rise to malfunction of the anode, and clogging of the pores in the cell membrane separator which leads to loss of ion transport and large increases in internal resistance in the cell.
In response to these challenges, new electrolytes [Ref. 12-15] and protective films [Ref. 16-18] for the lithium anode have been developed. Combinations of electrolyte modification, additives and anode protection have resulted in some promising results [Ref. 19]. However, much of the difficulty still remains at the cathode. For instance, in some cell configurations, all of the sulfides are allowed to be solubilized (so-called ‘catholyte’ cells) [Ref. 20]. In the opposite approach (i.e. to contain the sulfides), some interesting cathode developments have been reported recently [Ref. 21-24]; but, their performance still fall short of what is required for practical applications.
These studies include, for example, the fabrication of disordered mesoporous carbon/sulfur composites in conjunction with ionic liquid electrolytes; systems that achieve high initial capacity, but suffer extensive capacity fading [Ref. 24]. Composites with sulfur embedded in conducting polymers have shown some promising results [Ref. 25]. However, a large polarization was observed, resulting in a very low operating voltage that reduces the energy density of cells. The loading of active mass in the S-polymer composite is also limited (less than 55 wt %) owing to the low surface area of the conducting polymer.
Using a composite structure approach, Oyama et al., in U.S. Pat. No. 5,324,599, discloses composite cathodes containing disulfide organo-sulfur or polyorgano-disulfide materials, as disclosed by Dejonghe, et al. in U.S. Pat. No. 4,833,048, by a combination with or a chemical derivative with a conductive polymer. The conductive polymers are described as capable of having a porous structure that holds disulfide compounds in their pores.
In a similar approach to overcoming the dissolution problem with polyorgano-disulfide materials by a combination or a chemical derivative with a conductive, electroactive material, U.S. Pat. No. 5,516,598 to Visco et al. discloses composite cathodes comprising metal-organosulfur charge transfer materials with one or more metal-sulfur bonds, wherein the oxidation state of the metal is changed in charging and discharging the positive electrode or cathode. The metal ion provides high electrical conductivity to the material, although it significantly lowers the cathode energy density and capacity per unit weight of the polyorgano-disulfide material. This reduced energy density is a disadvantage of derivatives of organo-sulfur materials when utilized to overcome the dissolution problem. The polyorganosulfide material is incorporated in the cathode as a metallic-organosulfur derivative material, similar to the conductive polymer-organosulfur derivative of U.S. Pat. No. 5,324,599, and presumably the residual chemical bonding of the metal to sulfur within the polymeric material prevents the formation of highly soluble sulfide or thiolate anion species.
Most recently, Ji, et al [Ref. 26] reported that cathodes based on nanostructured sulfur/mesoporous carbon materials could overcome these challenges to a large degree, and exhibit stable, high, reversible capacities (up to 1,320 mAh per gram of the cathode material) with good rate properties and cycling efficiency. However, the fabrication of the proposed highly ordered mesoporous carbon structure requires a tedious and expensive template-assisted process.
Despite the various approaches proposed for the fabrication of high energy density rechargeable cells containing elemental sulfur, organo-sulfur and carbon-sulfur cathode materials, or derivatives and combinations thereof, there remains a need for materials and cell designs that retard the out-diffusion of anionic reduction products, from the cathode compartments into other components in these cells, improve the utilization of electroactive cathode materials and the cell efficiencies, and provide rechargeable cells with high capacities over a large number of cycles.
Most significantly, lithium metal (including pure lithium, alloys of lithium with other metal elements, or lithium-containing compounds) still provides the highest anode specific capacity as compared to essentially all anode active materials (except pure silicon, but silicon has pulverization issues discussed above). Lithium metal would be an ideal anode material in a lithium metal or lithium-air secondary battery if dendrite related issues could be addressed. In addition, there are several non-lithium anode active materials that exhibit high specific lithium-storing capacities (e.g., Si, Sn, and Ge as an anode active material) in a lithium ion battery wherein lithium is inserted into the lattice sites of Si, Sn, or Ge in a charged state.
Hence, an object of the present invention was to provide a rechargeable Li-metal or Li-ion battery that exhibits an exceptionally high specific energy or energy density. One particular technical goal of the present invention was to provide a Li metal-sulfur or Li ion-sulfur cell with a specific energy greater than 400 Wh/Kg, or even greater than 600 Wh/Kg.
A specific object of the present invention was to provide a Li metal or lithium-ion secondary cell wherein the cathode initially comprises an integrated structure of conductive nano-filaments with lithium sulfides (LixS8, partially or fully oxidized state of sulfur) dispersed in spaces (pores) between nano-filaments and wherein lithium sulfides are in fine powder form or a thin coating bonded to or coated on nano-filament surfaces. During the first charge operation, lithium sulfides are reduced to sulfur. During subsequent charge and discharge operations, lithium sulfides are essentially retained in the pores constituted by the nano-filaments, permitting only Li+ ions to diffuse back and forth between the anode and the cathode.
Another object of the present invention was to provide a simple (not too complex), cost-effective, and easier-to-implement approach to preventing potential Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal-sulfur batteries featuring the aforementioned nano-structured cathode.
Another object of the present invention was to provide a nano-structured composition for use as an anode substrate of a Li metal-sulfur cell that is resistant to dendrite formation and exhibits a long and stable cycling response. This nano-structured anode is assembled together with a nano-structured cathode.
Still another object of the present invention was to provide a lithium metal cell or Li-ion that exhibits a high specific capacity, high specific energy, good resistance to dendrite formation, and a long and stable cycle life.
Yet another object of the present invention was to provide a lithium-ion cell wherein the anode comprises a nano-structured composition (integrated structure of conductive nano-filaments and high-capacity anode active material such as Si particles, nano-wires, or thin coating) and the cathode comprises an integrated structure of conductive nano-filaments. The integrated structure, being highly conductive, can also function as a current collector, obviating the need to have a separate current collector at either the anode or the cathode side. Such a configuration can significantly reduce the overhead weights of a cell, thereby affording an ultra-high specific capacity and specific energy.